Are you using a HR strap?
I am having a similar issue:O
https://forums.garmin.com/showthread.php?377835-VO2-Max-not-updating-to-Connect
I'm not using a HR strap. I understand it is not necessary on the fenix 5 for training status updates. The device says to run a couple of times outdoors. I've clocked in 10 runs so far
I think it's required for a VO2max, but not sure about training status.
It's not! I have logged 18 runs using only watch HR (optical) and I get fine V02max readings!